FIG. 1 is constitutional view of a conventional excitation control device. In FIG. 1, 1 indicates a synchronous machine. 2 indicates a transformer. 3 indicates a circuit breaker. 4 indicates a power transmission line. 5 indicates a power transmission bus line. 6 indicates a potential transformer (hereinafter, called PT) for detecting a voltage VG of an output terminal of the synchronous machine 1.7 indicates a current transformer (hereinafter, called CT) for detecting a reactive current IQ output from the synchronous machine 1.8 indicates a voltage setting device for setting a reference voltage VGref of the output terminal of the synchronous machine 1 according to both the reactive current IQ detected in the CT 7 and a reference voltage VHref of the high voltage side of the transformer 2.
9 indicates a subtracting unit for subtracting the output terminal voltage VG detected in the PT 6 from the reference voltage VGref set in the voltage setting device 8 to obtain a subtraction value and outputting a difference signal indicating the subtraction value. 10 indicates an automatic voltage regulating device (hereinafter, called AVR) for controlling a commutation timing of an exciter 11 by using the difference signal output from the subtracting unit 9 as an input condition for a transfer function. 11 indicates the exciter for supplying a field current to a field winding 12 of the synchronous machine 1 according to an instruction of the AVR 10. 12 indicates the field winding of the synchronous machine 1.
FIG. 2 is a flow chart showing a conventional excitation control method.
Next, an operation will be described.
A voltage VG of the output terminal of the synchronous machine 1 is detected in the PT 6 (step ST1), and a reactive current IQ output from the synchronous machine 1 is detected in the CT 7 and the PT 6 (step ST2).
When the reactive current IQ is detected in the CT 7, a reference voltage VGref of the output terminal of the synchronous machine 1 is set in the voltage setting device 8 according to both the reactive current IQ and a reference voltage VHref of the high voltage side of the transformer 2 (step ST3).
Hereinafter, a setting method of the reference voltage VGref is described.
A relation between the voltage VG of the output terminal of the synchronous machine 1 and a voltage VH of the high voltage side of the transformer 2 is expressed according to an equation (1).VG=VH+Xt×IQ  (1)Here, the symbol Xt in the equation (1) denotes a reactance of the transformer 2.
Also, as shown in FIG. 3, in cases where a plurality of synchronous machines 1 are connected to a power transmission system, reactance of each synchronous machine 1 with another synchronous machine 1 is equal to almost zero by applying the equation (1) as a relation between the reference voltage VGref and the reference voltage VHref, a cross current flows from one synchronous machine 1 to another synchronous machine 1 due to both a voltage difference of the output terminal voltages VG and a response difference in each synchronous machine 1 with another synchronous machine 1, and each synchronous machine 1 has an excessive load. To suppress the generation of the cross current, as is expressed according to an equation (2), a reactance XDR corresponding to the suppression of the cross current is subtracted from the reactance Xt of the transformer 2. Here, the reactance XDR corresponding to the suppression of the cross current is set to a value equal to several % of the reactance Xt of the transformer 2, and the value of the reactance XDR is empirically set.VGref=VHref+(Xt−XDR)×IQ  (2)
Therefore, the reference voltage VGref of the output terminal of the synchronous machine 1 is calculated in the voltage setting device 8 by substituting the reactive current IQ output from the synchronous machine 1 and the reference voltage VHref of the high voltage side of the transformer 2 into the equation (2).
When the reference voltage VGref of the output terminal of the synchronous machine 1 is set in the voltage setting device 8, the voltage VG of the output terminal of the synchronous machine 1 detected in the PT 6 is subtracted in the subtracting unit 9 from the reference voltage VGref set in the voltage setting device 8 to obtain a subtraction value, and a difference signal indicating the subtraction value is output (step ST4).
When the difference signal is output from the subtracting unit 9, a timing signal for controlling a commutation timing of the exciter 11 is produced in the AVR 10, for example, by using the difference signal as an input condition for a following transfer function (step ST5).Transfer Function=K×(1+TLD×s)/(1+TLG×s)  (3)Here, the symbol K denotes a gain constant, the symbols TLD and TLG denote time constants, and the symbol s denotes a Laplace operator.
When the timing signal output from the AVR 10 is received in the exciter 11, a field current is supplied to the field winding 12 of the synchronous machine 1 according to the timing signal (step ST6). Here, when the difference signal output from the subtracting unit 9 is equal to a positive value, the field current supplied to the field winding 12 is increased, and the voltage VG of the output terminal of the synchronous machine 1 is heightened. In contrast, when the difference signal output from the subtracting unit 9 is equal to a negative value, the field current supplied to the field winding 12 is decreased, and the voltage VG of the output terminal of the synchronous machine 1 is lowered.
Therefore, the voltage VG of the output terminal of the synchronous machine 1 is controlled so as to agree with the reference voltage VGref. Also, when the reactive current IQ output from the synchronous machine 1 is equal to zero, the voltage VH of the high voltage side of the transformer 2 is controlled so as to agree with the reference voltage VHref.VG=VHref+(Xt−XDR)×IQ  (4)VH=VHref−XDR×IQ  (5)
Therefore, because the voltage of the power transmission bus line 5 is maintained to a constant value, even though a failure occurs, for example, in the power transmission line 4, the lowering of the voltage in the whole power transmission system can be lessened.
Because the conventional excitation control device has the above-described configuration, even though a failure occurs in the power transmission line 4, the lowering of the voltage in the whole power transmission system can be lessened. However, because no means for quickening the attenuation of an electric power fluctuation of power transmission system occurring due to a failure of the power transmission system is arranged in the conventional excitation control device, a problem has arisen that it is required to additionally arrange a power system stabilization control device (PSS) for the purpose of quickening the attenuation of an electric power fluctuation.
The present invention is provided to solve the above-described problem, and the object of the present invention is to provide an excitation control device and an excitation control method in which the attenuation of an electric power fluctuation is quickened while controlling a voltage of the high voltage side of a transformer to a constant value.